Ask how often the walkthrough happens and the answer is often "monthly." It goes on the calendar the same day every month, and the checklist says "monthly" across the top.
The federal rule says every 30 days. Those two schedules agree in only four months of the year. In the other eight, they pull apart by a day or two. One day can be the difference between a complete file and a gap.
The federal rule counts days, not months
The walkthrough rule is 40 CFR 280.36. It sets two clocks.
Every 30 days, you check the spill bucket for damage, liquid and debris. You confirm the release detection equipment shows no alarms and its records are current. One exception: if a tank gets fuel less often than every 30 days, the spill bucket check can happen before each delivery instead.
Once a year, you check the containment sumps and any hand-held release detection gear, like gauge sticks.
Part 280 uses "monthly" elsewhere, for inventory control and some older piping, but not for the walkthrough. Its definitions section does not define "monthly" at all.
A same-date schedule runs late seven times a year
The gap between the 15th of one month and the 15th of the next is the length of the first month. Seven months have 31 days. So a same-date schedule runs 31 days, one day past the federal count, seven times a year.
Take a walkthrough done on Tuesday, December 15, 2026. Thirty days later is Thursday, January 14, 2027. The same date next month is Friday, January 15. That is day 31.
February cuts the other way
Now take a walkthrough done on Saturday, January 30, 2027. Thirty days later is Monday, March 1. February 2027 has only 28 days, so a strict 30-day count skips it. No walkthrough lands in February at all.
That matters if your state's rule says "monthly" and someone reads it as "once in every calendar month." The same-date habit would have put it on February 28.
So neither count is safe on its own. The safe date is the earlier of the two.
UST Compliance Tools
Walkthrough Date Calculator
The federal rule counts every 30 days. Plan your next walkthrough so it lands inside that count, or check the days between two you already did.
Enter a date to see when the next walkthrough is due.
Every 30 days
The federal count, 40 CFR 280.36(a)(1)(i).
Same date next month
How most calendars count "monthly."
Plan for
Already did the next one?
| # | Every 30 days | Same date every month | Days since the last one (same date) |
|---|
Assumes each walkthrough is done on its due date. The same-date column keeps your original day of the month and uses the month's last day when that day does not exist.
Enter two walkthrough dates from your records to count the days between them.
Your second date is earlier than the first, so the count runs from the earlier date to the later one.
| # | From | To | Days between |
|---|
This checks the federal 30-day count. States word the rule differently, so check what your state's rule says.
The record is what gets counted
The rule also says what your walkthrough record must show (280.36(b)):
Each area you checked.
Whether each area was fine or needed action.
What you did to fix any problem.
Delivery records, if you checked the spill bucket less often than every 30 days because deliveries were rare.
Keep these records for one year under federal rules, at the site or somewhere you can produce them when the agency asks (280.34). The dates on them are how anyone reviewing your file counts the gaps.
States say it four different ways
State rules do not all use the federal words. A few examples from the rule text:
New York: "every 30 days," and keep walkthrough records for three years, not one (6 NYCRR 613-2.2(h)).
Hawaii: "every thirty-one days" (HAR 11-280.1-36(a)).
Florida: "at least once a month, but not exceeding 35 days" (62-761.600(1)(e), F.A.C.).
Maryland: "on a monthly basis" for the spill bucket check (COMAR 26.10.04.03).
Vermont: "monthly," and weekly at unstaffed sites (Chapter 8, 8-509(a)).
None of these define "monthly" for you. Some local programs add their own rules on top. If more than one rule reaches your tanks, meet the strictest one. Your state UST program can tell you which rules apply to your sites.
How to schedule by the strictest reading
Walk every 4 weeks, on the same weekday. That is every 28 days. Twenty-eight days is always inside the 30-day count. It is also shorter than every calendar month, so no month ever gets skipped. That meets both readings, all year. It is easy to staff, too, because it always falls on the same day of the week. The cost is one or two extra walkthroughs a year.
If you must schedule by calendar date, plan for the earlier date. Thirty days after the last walkthrough, or the same date next month, whichever comes first. That meets both readings.
Don't plan for the last day. Aim for day 25 to 28. Weather and staffing eat days.
Date and sign every record. A walkthrough with no date can't prove the gap.
Keep the annual items on their own clock. The sump and hand-held checks are yearly. Don't let them ride on the monthly sheet and get skipped.
Keep records longer than the minimum. One year is the federal floor. New York wants three.
Found a gap? Don't backdate it
If you find a gap in your records, never backdate a record. Never fill in a walkthrough that didn't happen. Do the walkthrough now and write down the real date. Add a short note saying why the gap happened. Then get back on schedule. A one-day gap, honestly recorded, reads far better to an inspector than a record that doesn't match what really happened.
The Bottom Line
"Every 30 days" is a count of days. A month is not. A same-date schedule goes to day 31 seven times a year. A strict 30-day count can skip February. Walk every 28 days, or plan each walkthrough for the earlier of the two dates. Write the date on the record, and keep the record. That is the whole fix.
Then check your own file. Pull your last 12 walkthrough records and run each pair of dates through the calculator above. Every gap over 30 days is a question an inspector can ask. Better that you find it first.
Federal rules require Class B operator training to cover operation and maintenance, recordkeeping and inspections (280.242(b)). If your team needs a refresher, the PASS Training Marketplace has state-specific Class A/B operator training.
Not sure which count your sites answer to? Ask us.
